Frictional forces exerted by blood flow on the vascular endothelium play an essential role in atherosclerosis, as well as aneurysm growth and pathological vascular remodeling. The endothelium expresses a similar profile of atheroprone, pro-inflammatory genes in response to low and oscillatory flow in all of these responses, suggesting fundamental molecular similarities. Although some of the immediate shear responsive signaling pathways have been identified, the downstream signaling which coordinates the vascular response remains unclear. Since vascular disease, the greatest cause of morbidity and mortality in the United States, is primarily caused by the formation of atherosclerotic plaques, understanding the mediators of the vascular response to altered blood flow is of critical importance. The extracellular matrix proteins underlying the endothelium dictat the inflammatory response to altered flow. A specific form of the secreted fibronectin protein, generated by alternative splicing of the constitutive transcript and normally absent in quiescent adult vasculature, is highly expressed in atherosclerotic lesions and other inflammatory vascular pathologies, such as aneurysm and neointima formation. Genetic mutations which prevent the formation of this isoform impair atherosclerosis in mice, but how this splicing is regulated and it biological consequences remain unclear. I hypothesize that alternative splicing of fibronectin in the endothelium is increased in response to atheroprone blood flow, and that the inclusion of alternative exons promotes atherosclerosis progression by promoting inflammatory cell recruitment and activation. To test this I will (1) Examine alternative splicing of fibronectin in response to atheroprone flow in vivo by molecular analysis of carotid arteries experimentally exposed to increased or decreased blood flow. I will then (2) Test the effect of deficient fibronectin splicing on monocyte recruitment/differentiation in the carotid arteries of mice unable to produce specific (EIIIA and EIIIB) fibronectin isoforms. Finally, I will (3) Identify the regulaors of alternative splicing in the endothelial response to flow by using barcoded shRNA to screen endothelial cells expressing a fluorescent FN splicing reporter in vitro.